Several important anticancer drugs act by disrupting the structure and function of microtubules. Unfortunately, the clinical utility of these agents is often limited by the development of tumor cell resistance to the drugs, necessitating the identification of new agents with greater efficacy toward drug resistant tumors. We have recently demonstrated that a novel cyanobacterial natural product named cryptophycin potently induces the depolymerization of microtubules in cultured cells. Importantly, cells which overexpress the drug transporter P-glycoprotein are strongly resistant to several antimicrotubule agents, including the vinca alkaloids, colchicine, rhizoxin and taxol, but are not resistant to cryptophycin. Therefore, cryptophycin may be useful as a cytotoxic agent in the therapy of drug-resistant tumors. In studies conducted under the present proposal, we will characterize the biochemical mechanism of action of cryptophycin, and assess its activity in vivo, including its efficacy against drug-resistant tumors. Mechanistic studies on cryptophycin will include synthesis of radiolabeled analogues, characterization of its effects on microtubule assembly and disassembly in vitro; and analysis of its interactions with tubulin and other microtubule-associated proteins. The abilities of several drug- resistance systems to confer cellular resistance to cryptophycin will be assessed. Additionally, cryptophycin-resistant cell lines will be developed to examine cellular responses to this agent. In vivo pharmacological studies will include analysis of the biodistribution and toxicity profiles of cryptophycin in mice, and assessment of its effects on the growth of drug-sensitive and drug-resistant tumor cells. These preclinical studies of cryptophycin should characterize the molecular actions of this compound and allow preliminary assessment of its potential usefulness as an anticancer agent.